The invention relates to a lenticular device. In particular, but not exclusively, the invention relates to a lenticular device in the form of a lenticular sheet comprising first and second arrays of optical micro-elements, which together provide a Moirxc3xa9 pattern. Such a lenticular sheet may be provided in or with a record carrier such as an optical disk. The presence of an expected Moirxc3xa9 pattern from a given sheet indicates that the object with which the sheet is associated is genuine.
Lenticular sheets which provide Moirxc3xa9 patterns have previously been proposed for use as anti-counterfeit devices. A known lenticular sheet is illustrated in FIG. 1. The sheet comprises a layer 1 on a first side 2 of which is provided a lenticular array 4 of embossed convex microlenses 5 having a pitch t1,. On a second side 3 of the layer there is provided a lenticular array 6 of embossed convex microlenses 7 having a slightly different pitch t2. Since the microlenses of arrays 4 and 6 are out of register by t2xe2x88x92t1 the sheet provides a Moirxc3xa9 pattern of period (t1*t2)/|t1xe2x88x92t2|.
When viewed from the first side 2 the Moirxc3xa9 pattern appears as a repetition of enlarged images of the microlenses 7 on the second side 3. Since the optical micro-elements on the second side 3 are of circular convex microlenses the enlarged images in the Moirxc3xa9 pattern each possess depth.
Copying lenticular sheets is difficult. It is difficult to copy precisely the positional relationship of the arrays of a sheet. Flaws in the positional relationship of the arrays of a copied sheet compared to that of an original sheet will result in the Moirxc3xa9 pattern of the copied sheet being different to that of the original sheet. The difference between the patterns produced by a genuine sheet and an inaccurately copied sheet can be identified either by a naked eye or a verifying device. Forged objects carrying flawed copied lenticular sheets can therefore be identified.
It is not however impossible to copy lenticular sheets such as those illustrated in FIG. 1. By accurately mapping the positional relationship of the arrays of the sheet it is possible to produce moulds carrying the negative of the arrays. The moulds can then be used to mass produce accurately copied sheets for use with forged objects.
WO 99 23513 A describes a lenticular sheet the microlenses of one of the lenticular arrays can have a cross-section in a plane perpendicular to the corresponding surface which includes at least two discontinuities, providing an annular reflective portion in each of the microlenses of that array, thereby improving the visibility of the Moirxc3xa9 pattern viewed through the other array.
WO 94 27254 A describes a lenticular device including a lenticular array and an array of two dimensional micro-images. The lenticular array is fixed against, or may be brought up against, the array of micro images. The micro images are formed by a printing process, in which the printing plate is formed by the exposure of a photographic resist through a microlens array of form corresponding to the lenticular array. It should be noted that such printing of images provides a different visual effect than that of embossing, in which the visual pattern is formed by varying degrees of internal reflection at different angles of incidence, rather than contrast in the image.
U.S. Pat. No. 3,357,773 describes a lenticular sheet material having a lenticular array on one side of the material and a different lenticular array on the other side. Various types of Moirxc3xa9 patterns are produced by variously stretching one or the other of the sides of the material before the two sides are brought together. However, such stretching, which produces continuous variations in the pitch of at least one of the arrays, produces effects which are relatively easily reproduced and difficult to verify as genuine, and was intended only to provide various decorative effects.
It would be desirable to provide a lenticular device having verifiable features which render the device more difficult to copy.
According to one aspect of the invention there is provided a lenticular device comprising:
a first array of lenticular elements on a first surface of the device; and
a second array of lenticular elements on a second surface of the device,
wherein the first and second arrays are arranged such that, when the second array is viewed through the first array, a Moirxc3xa9 pattern showing an enlarged image representing elements of the second array is visible,
characterised in that at least one of the arrays includes at least one predetermined and verifiable discontinuous variation, the variation being in a characteristic of at least one of the elements of the array with respect to the same characteristic of an adjacent element of the array.
According to a another aspect of the invention there is provided a lenticular device comprising:
a first array of lenticular elements on a first surface of the device; and
a second array of elements on a second surface of the device, said second array of elements consisting of substantially homogeneous elements,
wherein the first and second arrays are arranged such that, when the second array is viewed through the first array, a Moirxc3xa9 pattern showing an enlarged image representing an element of the second array is visible,
characterised in that said second array includes at least one predetermined and verifiable discontinuous variation, the variation being in a characteristic of at least one of the elements of the array with respect to the same characteristic of an adjacent element of the array.
A discontinuous variation is a variation which does not occur smoothly across a length, or width, of the array. A discontinuous variation between adjacent elements cannot be achieved merely by stretching the material on which the array is formed.
According to another aspect of the invention there is provided a lenticular device comprising:
a first array of lenticular elements on a first surface of the device; and
a second array of lenticular elements on a second surface of the device,
wherein the first and second arrays are arranged such that, when the second array is viewed through the first array, a Moirxc3xa9 pattern showing an enlarged image representing an element of the second array is visible,
characterised in that at least one of the elements includes an embossment providing the at least one of the elements with a cross-section, parallel to the surface on which the at least one of the elements is formed, which has a predetermined and verifiable shape, said shape being irregular in comparison with that of a parallel cross-section through a part of the at least one element not including said embossment.
Such devices have structure which make the device more difficult to copy. Since the shape of the cross section provided by the embossment is predetermined, a flawed copy can be detected by a variation in the shape, while genuine articles can be readily verified, either by an unaided observer or by an aided observer, as will be appreciated in further detail from the following.
According to another aspect of the invention there is provided a lenticular device comprising:
a first array of elements on a first surface of the device, the elements thereof being lenticular; and
a second array of elements on a second surface of the device,
wherein the first and second arrays are arranged such that, when the second array is viewed through the first array, a Moirxc3xa9 pattern showing an enlarged image representing elements of the second array is visible,
characterised in that at least one predetermined and verifiable defect is encoded into at least one of the first and second arrays, which defect is substantially indiscernible to an unaided observer.
Such a device has a security structure in the array which makes the sheets difficult to copy but produces no directly visible effect. The presence or absence of the structure must be verified by a device such as a microscope.